1. Field of Invention
The invention relates to a hydraulic control circuit for a boom cylinder in work machine, such as a hydraulic excavator, that is used in various types of construction and civil engineering works.
2. Description of Related Art
Generally, in some construction and civil engineering machinery, such as a hydraulic excavator, there is a type in which a front attachment mounted on the machinery body is composed of a boom whose base end portion is supported so as to swing upward and downward, an arm that is supported at the tip end portion of the boom so as to swing forward and backward, and a work attachment, such as a bucket attached to the tip end portion of the arm. In such a type, the above-described boom moves upward and downward in line with extension and contraction of a boom cylinder, wherein, conventionally, a hydraulic control circuit, as shown in FIG. 3, has been known as a hydraulic control circuit for the boom cylinder.
That is, in FIG. 3, reference number 8 denotes a boom cylinder. Reference numbers 11, 12 denote the first and second pressurized oil supply sources. Reference number 13 denotes an oil reservoir. Reference number 14 denotes the first boom control valve for controlling the supply of pressurized oil to the boom cylinder 8 and discharge of pressurized oil therefrom, in which the first pressurized oil supply source is used as its pressurized oil supply source 11. Reference number 15 denotes the second boom control valve for controlling the supply of pressurized oil to the boom cylinder 8, in which the second pressurized oil supply source 12 is used as its pressurized oil supply source. Reference numbers 16 through 18 denote control valves for other hydraulic actuators, such as an arm cylinder and a bucket cylinder, attached to the hydraulic excavator. Reference number 19 denotes a pilot valve for outputting pilot pressure to the elevation side and descent side pilot lines C, D on the basis of operations of a boom operating lever 20. In addition, reference letter A denotes the head side line for connecting the first boom control valve 14 and the second boom control valve 15 respectively to the head side oil chamber 8a of the boom cylinder 8. Reference letter B denotes the rod side line for connecting the first boom control valve 14 to the rod side oil chamber 8b of the boom cylinder. Also, reference letter G denotes a recycling line for communicating the above-described head side line A and rod side line B with each other. In the recycling line G, a recycling valve 30 having a check valve 30a is provided.
In this structure, when the boom operating lever 20 is operated to the elevation side, the first and second boom control valves 14, 15 are changed to the elevation side position X by pilot pressure outputted from the pilot valve 19 to the elevation side pilot line C, wherein pressurized oil from both the first and second pressurized oil supply sources 11, 12 is supplied into the head side oil chamber 8a of the boom cylinder 8, and it is possible to efficiently carry out an upward motion (elevation) of the boom 5 against the weight of a front attachment.
On the other hand, when the boom operating lever 20 is operated to the descending side, the first boom control valve 14 is changed to the descending side position Y by pilot pressure outputted from the pilot valve 19 to the descending side pilot line D, and at the same time, the recycling valve 30 is changed to the second position Y where the recycling line G is opened, wherein, while the pressurized oil from the first pressurized oil supply source 11 is supplied into the rod side oil chamber 8b of the boom cylinder 8 via the first boom control valve 14, the oil discharged from the head side oil chamber 8a is discharged into the oil reservoir 13 via the first boom control valve 14 and, at the same time, is further supplied into the rod side oil chamber 8b via the recycling valve 30. That is, when the boom descends, while the pressure of the head side oil chamber 8a is higher than that of the rod side oil chamber 8b, the oil discharged from the head side oil chamber 8a may be supplied into the rod side oil chamber 8b as the recycling oil, wherein the recycling oil is supplied into the rod side oil chamber 8b in addition to the pressurized oil of the first pressurized oil supply source 11, which is supplied from the above-described first boom control valve 14, and accordingly the operation speed of the boom cylinder 8 can be made fast with the rod side oil chamber 8b not placed in a pressure-reduced state. Also, because a surplus pump oil flow obtained by recycling can be supplied into other hydraulic actuators when a combined operation including operation of the other hydraulic actuators (for example, an arm cylinder and a bucket cylinder), for which the pressurized oil supply source for the boom cylinder 8 is concurrently used, and descent of the boom is carried out, it is possible to prevent the operation speed of the other hydraulic actuators from being lowered in a combined operation. Therefore, the structure contributes to an improvement in the work efficiency.
However, where the above-described boom is caused to descend to carry out surface compaction work and scraping work of an inclined plane by descent of the boom, because a force against the descent of the boom operates, it is necessary to supply highly pressurized oil into the rod side oil chamber 8b. To the contrary, where the boom is caused to descend in the air (that is, where the boom descends with the front attachment not grounded), as the weight applied to the boom (that is, the total weight of the front attachment) operates as a force for contraction of the boom, pressurized oil that is supplied into the rod side oil chamber 8b may be of low pressure. Further, because the head side area of the piston of the boom cylinder is larger than the rod side area, only the recycling oil from the above-described head side oil chamber 8a may be sufficient.
Therefore, in the above-described prior art hydraulic circuit, even if the boom is caused to descend in the air, not only the recycling oil but also pressurized oil from the first pressurized oil supply source are supplied via the first boom control valve 14. Accordingly, where the arm and bucket are operated while causing the boom to descend in the air, the pressurized oil from the first pressurized oil supply source 11 is shared by the boom cylinder, arm cylinder and bucket cylinder, wherein motions of the arm and bucket become slow in comparison with independent operations thereof, and there is a problem in that work efficiency is worsened. Further, in the case of causing the boom to independently descend in the air, because pressurized oil from the first pressurized oil supply source is supplied into the rod side oil chamber 8b in spite of only the recycling oil from the head side oil chamber 8a being sufficient, a considerable amount of surplus oil of the oil discharged from the head side oil chamber 8a is discharged into the oil reservoir 13 via the first boom control valve 14, wherein there is another problem in that energy loss is brought about, which may hinder improvement in fuel efficiency. These are objects to be solved by the invention.
In addition, in a work machine provided with a front attachment consisting of the above-described boom, arm and work attachment, etc., for example, wherein debris rake-up work is carried out with the bottom of the bucket grounded while moving the boom forward and backward, although three operations of the boom, arm and bucket are obliged to be carried out at the same time for the boom to depict a roughly horizontal locus, the operations are delicate, and skilled operations are required. In addition, where the ground is hardened by continuously repeated operations of descent and elevation of the boom, that is, bumping work is carried out, unless the boom elevation operation is carried out at the instant when the bucket bottom is grounded, the ground is excessively bumped by a reaction caused by the descent of the boom, or the front part of a machine body is raised. Therefore, in order to continuously carry out bumping, a considerably skilled operation is required. Work requiring such skill is difficult for a beginner, and even a skilled operator is obliged to pay meticulous attention to the work. Accordingly, another problem to be solved exists in that, in such situations, the operationality and work efficiency are made worse.